1. Field of the Invention
The present invention relates to an acoustic apparatus.
2. Description of the Related Art
It is known that when sound waves of a natural frequency are emitted in space surrounded by walls of acoustic equipment, standing waves are produced by the reciprocal motion of the sound waves between the wall surfaces of the space, which affect the acoustic characteristics of the acoustic equipment. Japanese Patent No. 2606447, Japanese Patent No. 3763682, and Japanese Patent Application Laid-open No. 2008-131199 disclose techniques of suppressing standing waves in a loudspeaker which is one type of acoustic equipment. A speaker apparatus disclosed in Japanese Patent No. 2606447 includes a speaker unit, a cabinet housing the speaker unit, and a Helmholtz resonator provided in the cabinet. A neck length L and a cavity volume V of the Helmholtz resonator in the speaker apparatus are designed in such a way that the Helmholtz resonator resonates at the same frequency as that of standing waves present in the cabinet. When a standing wave is produced in the cabinet of this speaker apparatus, the resonance phenomenon of the Helmholtz resonator occurs, attenuating the standing wave. A speaker apparatus disclosed in Japanese Patent No. 3763682 includes a speaker unit, a cabinet housing the speaker unit, and an acoustic tube (closed tube) having an open end and a closed end. The acoustic tube of the speaker apparatus has a tube length L which is a quarter of a wave length corresponding to the lowest resonance mode of a standing wave produced in the cabinet. This acoustic tube is housed in the cabinet in such a state where the position of its open end comes close to the position of the anti-node (node of the particle velocity) of the sound pressure of the standing wave in the cabinet. When a standing wave (whose wavelength is four times the tube length L) is produced in the cabinet of this speaker apparatus, a resonance wave is produced in the acoustic tube. This resonance wave has a node (anti-node of the particle velocity) of a sound pressure at the open end of the acoustic tube, and an anti-node (node of the particle velocity) of the sound pressure at the closed end. Accordingly, the speaker apparatus relaxes biasing of the distribution of the sound pressure in the cabinet, attenuating the standing wave in the cabinet. Japanese Patent Application Laid-open No. 2008-131199 also discloses a technique similar to the technique of Japanese Patent No. 3763682.